1. Technical Field
This invention relates in general to a direct access storage device (DASD) of the type utilizing dual spin valve magnetoresistive sensors for reading signals recorded in a magnetic medium and, more particularly, it relates to a DASD having a self-biased dual spin valve sensor.
2. Description of the Background Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced information tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
One type of MR sensor currently under development is giant magnetoresistive (GMR) sensors manifesting the GMR effect. In the GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., NiFe or Co or NiFe/Co) separated by a layer of GMR promoting non-magnetic metallic material (e.g., copper) are generally referred to as spin valve (SV) sensors. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or FeMn) layer. The pinning field generated by the antiferromagnetic (AFM) layer is usually equal or greater than 200 Oersteds (Oe) so that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other layer, referred to as the free layer (free magnetic layer), however, is fixed and is free to rotate in response to the field from the disk.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A free layer (free MR layer) 110 is separated from a pinned layer (pinned MR layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 125. Free layer 110, spacer layer 115, pinned layer 120 and the AFM layer 125 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the MR free layer 110 and the pinned layer 120. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source 160 to the MR sensor 100.
The SV effect, i.e., the net change in resistance, exhibited by a typical prior art SV sensor is about 3% to 4.5%. U.S. Pat. No. 5,206,590 entitled "Magnetoresistive Sensor Based On The Spin Valve Effect", granted to Dieny et al. on Apr. 27, 1993, discloses an MR sensor operating on the basis of the spin valve effect.
Referring to FIG. 2, there is shown another type of spin valve sensor commonly referred to as dual spin valve sensor 200, comprising end regions 204 and 206 separated by a central region 202. A free layer (free MR layer) 210 is separated from two outer pinned (pinned MR layer) layers 220 (PL1) and 230 (PL2) by two SV promoting spacer layers 240 and 250, respectively. The magnetization of the first pinned layer 220 is fixed through exchange coupling with a first antiferromagnetic (AFM1) layer 225. The magnetization of the second pinned layer 230 is fixed through exchange coupling with a second antiferromagnetic (AFM) layer 235 (AFM2). Free layer 210, spacer layers 240 and 250, pinned layers 220 and 230 and the AFM layers 225 and 235 are all formed in the central region 202. Hard bias layers 260 and 270 formed in the end regions 204 and 206, respectively, longitudinally bias the MR free layer 210. Leads 280 and 290 formed over hard bias layers 260 and 270, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source (not shown) to the MR sensor 200.
Referring to FIG. 2, the magnetization directions of PL1 and PL2 are arranged in such a way that a rotation of the free layer magnetization generates an SV signal of equal sign across the SV promoting spacers, leading to a larger SV effect in dual spin valve sensors as compared to the SV effect in the spin valve sensor of FIG. 1. For example, a dual spin valve sensor having a 70 angstrom thick free layer exhibits an SV effect of about 3.6%-5.5% in comparison to an SV effect of about 3%-4.5% for a single spin valve sensor of the same thickness free layer.
However, there are several significant problems associated with the typical dual SV sensor of FIG. 2.
First, a typical prior art dual SV sensor requires two antiferromagnetic layers (AFM1 and AFM2)with sufficiently large exchange bias fields (usually greater than 200 Oe) in order to pin PL1 and PL2. However, in practice it is difficult to have sufficiently large exchange bias fields for pinning the pinned layers because exchange bias fields magnitude changes significantly depending on whether the ferromagnetic layer is deposited on the AFM layer (PL1 220 on AFM1 225) or the AFM layer is deposited on the ferromagnetic layer (AFM2 235 on PL2 230);
second, it is known that in a conventional spin valve sensor (FIG. 1), the stray field from the pinned layer causes a nonuniform magnetization distribution in the free layer leading to approximately 30% reduction of the linear portion (also referred to as the "usable portion" or "dynamic range") of the SV effect. The added stray field from the second pinned layer in the dual spin valve sensor reduces the SV linear range by approximately 60%, largely offsetting the SV effect advantage of the dual spin valve sensor; and PA1 third, prior art dual spin valve sensors require two AFM layers made of FeMn or NiO in order to pin the magnetization of both pin layers by creating pinning fields of about 200 Oe. However, both FeMn and NiO have rather low blocking temperatures (blocking temperature is the temperature at which pinning field for a given material reaches zero Oe) which make their use as an AFM layer difficult and undesirable. Referring to FIG. 3, there is shown the change in the pinning field versus temperature for FeMn having the blocking temperature of about 150.degree. C. (line 310) and NiO having the blocking temperature of about 200.degree. C. (line 320). Considering that a typical SV sensor used in a DASD should be able to operate at a constant temperature of about 120.degree. C. at a pinning field of about 200 Oe, it can readily be seen that FeMn substantially loses it ability to pin the pinned layer at about 120.degree. C. and NiO can marginally provide adequate pinning at about 120.degree. C. It should be noted that once the pinning effect is lost, the SV sensor loses its SV effect either totally or partially rendering the SV sensor useless. FeMn is also very prone to corrosion which makes its use as an AFM layer even more problematic than NiO.
Therefore, there is a need for an invention in which pinned layers' magnetization cancel each other out and further discloses a means for pinning the magnetization of the pinned layers in a dual SV sensor without utilizing an antiferromagnetic layer, thus eliminating the dual SV sensor operation on the temperature sensitivity of the AFM layers.